In most conventional internal combustion engines, the intake manifold(s) is/are the volume(s) between the throttle(s) and working chamber intake ports. Air flows through the intake manifold from the ambient environment to the working chambers. When a driver depresses the accelerator pedal, a throttle valve is opened to allow more air into the intake manifold. The resulting increase in the manifold absolute pressure (MAP) causes more air to enter the working chambers and increases engine output.
Since a conventional vehicle is frequently operating at much less than full throttle, the MAP tends to be much lower than atmospheric pressure (i.e., for engines that are not boosted). In other words, there tends to be a substantial vacuum in the intake manifold. This vacuum can be used for a variety of other purposes, as will be described in more detail below in connection with FIG. 1.
FIG. 1 illustrates an internal combustion engine that includes a crankcase 167, a cylinder 161, a piston 163, an intake manifold 165 and an exhaust manifold 169. The fuel tank 151 is connected via a line to a fuel vapor canister 155, which in turn is connected to the intake manifold 165. A brake vacuum booster reservoir 157 is also connected to the intake manifold. The throttle valve 171 controls the inflow of air from an air filter or other air source into the intake manifold.
The fuel in fuel tank 151 is volatile and generates fuel vapor that, for environmental reasons, should not be released into the ambient environment. As a result, the vehicle includes a fuel vapor canister 155 that contains a suitable absorber material 153 (such as charcoal) for capturing the fuel vapor. A vacuum vapor line 154 connects the fuel vapor canister 155 to the intake manifold 165 through a fuel vapor canister valve 156. When the fuel vapor canister valve 156 is open vacuum within the intake manifold 165 is used to draw the fuel vapor through the vapor line 154 into the intake manifold 165. From the intake manifold, the fuel vapor passes into the working chambers of the engine, where it is combusted and passed through the exhaust manifold 169 to the catalytic converter.
It is also desirable to remove vapor that collects in the crankcase 167. During the operation of the engine, gases (both burned and unburned) leak from the cylinders into the crankcase past the piston rings (not shown in FIG. 1). These gases must be vented to avoid pressure buildup in the crankcase. For environmental reasons it is desirable if these gases are vented through the intake manifold into the cylinders, since the gases may contain combustion byproduct and hydrocarbons. The gases may be vented into the intake manifold using the positive crankcase ventilation valve (PCV) 173 or directly in a crankcase ventilation (CCV) system. The PCV valve is a one way valve that only allows flow from the crankcase 167 into the intake manifold 165. Vacuum in the intake manifold is helpful to remove these noxious gases from the crankcase.
Power braking systems also make use of a vacuum in the intake manifold 165. More specifically, the brake vacuum booster reservoir 157 is a canister that includes a diaphragm. When the driver presses the brake pedal, air is allowed to enter on one side of the diaphragm. There is a vacuum in a low pressure region on the other side of the diaphragm, which is maintained through a connection 175 with the intake manifold. This pressure differential amplifies the force that is applied to the brake pedal and increases braking power.